Sulfonylurea-containing compounds are known antitumor agents. See, e.g., U.S. Pat. No. 5,234,955 and U.S. Pat. No. 5,262,440. As these compounds have been found to accumulate in the mitochondria of cells, the mitochondria may be their target site of these compounds. Hydroxyguanidine-containing compounds have also been reported to possess potent antitumor activities. See Adamson, et al., Nature 1972, 236, 400 and Tai, et al. J. Med. Chem. 1984, 27, 236. The cellular target of hydroxyguanidine-containing compounds, however, is presumed to be ribonucleotide reductase.